Heat resistant polyamide-imide resin from polycarboxylic acid-polyisocyanate modified with unsaturated monomer

ABSTRACT

A heat resistant resin composition comprising a polyamide imide resin having carboxyl groups and modified with an N-methylolated methacryl- or acryl-amide compound and, when desired, further with a bismaleimide compound. The composition has improved properties over conventional polyamide imide resins. When the composition is used, for example, in the form of a film for printed circuit boards, the film has improved heat resistance and enhanced flexural strength and is capable of withstanding soldering baths at high temperatures.

This invention relates to polyamide imide resins known as heat resistantresins. The object of the invention is to improve the heat resistanceand/or flexural strength of polyamide imide resins.

The polyamide imide resins heretofore well known are excellent inelectrical characteristics, heat resistance and mechanical propertiesand are therefore used for electrical insulating varnishes and heatresistant films. It is desired that films for printed circuit boards,for example, be capable of withstanding a soldering bath at 260° to 280°C., whereas the polyamide imide resins commercially available at presentstill remain to be improved in their heat resistance in such a solderingbath.

We have found that when a polyamide imide resin having carboxyl groupsis modified with a compound selected from N-methylolated methacryl- oracryl-amide and derivatives thereof, the resulting resin has improvedresistance to heat while retaining the desired electricalcharacteristics and affords films which have enhanced flexural strengthin some cases. Based on this finding, the present invention has beenaccomplished. Although it appears useful to use a severe curingtemperature for imparting improved heat resistance to polyamide imideresin containing carboxyl groups, we have found that the carboxyl groupthen undergoes condensation with the hydrogen atom on the nitrogen ofthe intermolecular amide linkage, increasing crosslinked portion causingseriously impaired flexural strength of the film thereof. However, thecarboxyl group, when condensed with N-methylolated methacryl- oracryl-amide, permits highly reactive acrylol groups to cross-link witheach other more easily without entailing a reduction in the molecularweight of the cross linked portion. This makes it possible to giveimproved heat resistance to the resin without deteriorating the flexuralstrength thereof.

According to the invention, useful polyamide imide resins are thosehaving carboxyl groups. Such resins can be prepared by variousprocesses. Among the polyamide imides prepared from polybasic acidanhydrides and diisocyanates, those produced from aromatic polybasicacid anhydrides and aromatic diisocyanates are preferable especially forassuring improved heat resistance. For example, a suitable resin can beprepared by reacting an excess of trimetallitic anhydride with anaromatic diisocyanate at 50° to 200° C. for several hours. In this case,it is desirable to use 1.0 to 1.2 moles of trimellitic anhydride permole of the diisocyanate. Although a larger amount of the acid anhydrideis usable, use of a large excess of the anhydride affords only apolyamide imide polymer of low molecular weight which has insufficientstrength for use in the form of a film. Further in such a case it ispossible to react an excess of trimellitic anhydride or pyromelliticanhydride with a polyamide imide having a terminal isocyanate group andprepared by the process disclosed in Published Examined Japanese PatentApplication No. 16080/1967 to introduce carboxyl groups into thepolyamide imide resin.

For the preparation of polyamide imide resins having carboxylic groups,it is generally convenient to use an aromatic diisocyanate as adiisocyanate and an aromatic tribasic acid anhydride as a polybasic acidanhydride in the form of a mixture, which may further contain anaromatic tetrabasic acid anhydride as another polybasic acid anhydridewhen so desired. This yields a resin having good resistance to heat.

Examples of useful aromatic diisocyanates are tolyene diisocyanate,diphenylmethane diisocyanate, diphenylether diisocyanate, etc., whichare used singly or in admixture. Examples of useful aromatic polybasicacid anhydrides are trimellitic anhydride, pyromellitic anhydride,benzophenonetetracarboxylic anhydride, etc. Since the use of at least 50mole % of trimellitic anhydride affords good results in solubility, itis preferable to use pyromellitic anhydride orbenzophenonetetracarboxylic anhydride in an amount of up to 50 mole %,more preferably 0 to 40 mole %, based on total polybasic acid anhydrideemployed. The polycondensation reaction is conducted with use of asolvent. Exemplary of useful solvents are dimethylformamide,dimethylacetamide, dimethyl sulfoxide, hexamethylphosphoric amide,N-methylpyrrolidone and like aprotic polar solvents. These solvents areusable singly or in admixture. Aromatic hydrocarbons and ketones arealso usable conjointly with such solvents insofar as the polyamide imideresin does not separate out.

The polycondensation reaction is carried out preferably at a temperatureof 50° to 200° C., more preferably 100° to 180° C. The carboxyl groupsin the resulting polyamide imide resin are quantitatively determined bypotentiometric titration with use of sodium methylate.

Preferably the polyamide imide resin thus obtained contains at leastfour carboxyl groups per molecule. Resins containing a smaller number ofcarboxyl groups, although usable, will not be modified effectively withN-methylolated methacryl- or acryl-amide or derivatives thereof.

N-methylolated methacryl- or acryl-amide and derivatives thereof areused preferably in an amount of about 1.0 to about 5.0 mole equivalentweights, more preferably about 1.0 to about 2.0 mole equivalent weights,per mole equivalent weight of carboxyl groups.

Examples of useful amides are N-methylolated acrylamide,N-methoxymethoxymethylacrylamide, N-butoxymethylacrylamide,N-methylolated methacrylamide, N-butoxymethylmethacrylamide, etc.Although these compounds are usable singly or in admixture, the combinedamount of the compounds should not exceed 5 mole equivalent weights permole equivalent weight of the carboxyl groups. When these amidecompounds are used in excessive amounts, the film obtained will have lowheat resistance and poor flexural strength.

The polyamide imide having carboxyl groups is reacted withN-methylolated methacryl- or acryl-amide or a derivative thereof at atemperature of 50° to 200° C., preferably 100° to 150° C. for 0.5 to 3hours. At high temperatures, such amide compounds in the reaction systemare likely to polymerize, so that satisfactory results can be achievedif the reaction is effected in the presence of 5 to 200 ppm of apolymerization inhibitor such as BHT (butylated hydroxytoluene).

The polyamide imide resin obtained has the feature of being radiallycurable and possesses improved resistance to heat and also enhancedflexural strength as compared with the original resin.

We have also found that a resin of further improved heat resistance canbe obtained by adding a bismaleimide compound of the formula belowincluding derivatives thereof to the above reaction system for furthermodification. The bismaleimide compound can be used in the mixture withN-methylolated methacryl- or acryl-amide or a derivative thereof, andpolyamide imide resin to be modified, or can be added to the reactionmixture of polyamide imide resin modified with N-methylolated methacryl-or acryl-amide or a derivative. This finding has led to another featureof the invention. ##STR1## wherein X is CH₂, O, SO₂, S, or C(CH₃)₂.

The carbon-carbon double bond of bismaleimide copolymerizes with thecarbon-carbon double bond of N-methylolated methacryl- or acryl-amide orderivative thereof, so that the film obtained is transparent and hasoutstanding heat resistance.

Bismaleimide is used preferably in an amount of 5 to 50% by weight basedon the polyamide imide resin. With less than 5% by weight ofbismaleimide present, sufficiently improved heat resistance will notresult, whereas use of more than 50% by weight of the compound givesfilms of impaired flexural strength.

For the preparation of films, azobisisobutyronitrile,azobisvaleronitrile or like zero compound, or di-t-butyl peroxide,benzoyl peroxide or like peroxide can be added to the present resin topromote the formation of films. However, when films are formed at atemperature of not lower than 150° C., the desired film of polyamideimide resin can be obtained merely by heating in the absence of suchradical initiator.

The invention will be described in detail with reference to thefollowing examples, to which, however, the invention is not limited.

PREPARATION OF POLYAMIDE IMIDE RESIN I

A 0.95 mole quantity of diphenylmethane diisocyanate (MDI) and 1.00 moleof trimellitic anhydride were placed into a 2-liter three-necked flaskhaving its interior air replaced by nitrogen, and 1200 ml ofN-methyl-2-pyrrolidone was further placed into the flask. With stirring,the mixture was reacted at 100° C. for 1.5 hours, at 130° C. for 2 hoursand further at 180° C. for 6 hours.

The solution of the polyamide imide resin in N-methyl-2-pyrrolidone hada viscosity of 2,000 centipoises at room temperature.

PREPARATION OF POLYAMIDE IMIDE RESIN II

A 1.02 mole quantity of diphenylmethane diisocyanate and 1.00 mole oftrimellitic anhydride were placed into a 2-liter three-necked flaskhaving its interior air replaced by nitrogen, and 1200 ml ofN-methyl-2-pyrrolidone was further placed into the flask. The mixturewas reacted at 100° C. for 1 hour, at 130° C. for 2 hours and further at180° C. for 4 hours. Subsequently 0.04 mole of pyromellitic anhydridewas added to the mixture, and the resulting mixture was reacted at 130°C. for 2 hours.

A solution of the polyamide imide resin in N-methyl-2-pyrrolidine had aviscosity of 14,000 centipoises at room temperature.

EXAMPLES 1-3

N-methylolated acrylamide was added to a solution of the polyamide imideresin I, and the mixture was treated in the presence of 10 ppm of BHT at120° C. for 1 hour and thereafter cooled.

Benzoyl peroxide (2% by weight) was added to the resulting mixture, andthe resin solution obtained was applied to a glass plate. The coatingplate was heated to 180°-280° C. over a period of 30 minutes to preparea film.

The same procedure as above was repeated with use of the polyamide imideresin II solution and a commercial polyamide imide resin varnish(trademark "HI-400", produt of Hitachi Kasei Co., Ltd., Japan having aviscosity of 6,000 centipoises at room temperature) for Examples 2 and3, respectively.

The results are given in Table 1, which also shows the properties offilms produced from the resins I, II and HI-400.

The films were all 50 μ in thickness.

                                      TABLE 1                                     __________________________________________________________________________                Carboxyl                                                                           N--methylolated                                                                        Heat resistance *3                                                                     Flexural                                     Resin     groups *1                                                                          acrylamide *2                                                                          260° C.                                                                    280° C.                                                                     strength                                   __________________________________________________________________________    Polyamide imide resin I                                                                   About 25                                                                           --       c   c    15                                         Example 1   About 25                                                                           About 1.2                                                                              a   b    28                                         Polyamide imide resin II                                                                  About 16                                                                           --       b   c    25                                         Example 2   About 16                                                                           About 1.1                                                                              a   a-b  38                                         HI-400      About 20                                                                           --       b   c    21                                         Example 3   About 20                                                                           About 1.5                                                                              a   a-b  34                                         __________________________________________________________________________     Note:                                                                         *1 The number of carboxyl groups per unit of the polyamide imide.             *2 Mole equivalent weights per mole equivalent weight of carboxyl groups      in the polyamide imide.                                                       *3 The film was immersed in a soldering bath at 260° C. or             280° C. for 3 seconds. The mark "a" stands for no changes, "b"         stands for a film partially formed with irregular patterns, and "c"           indicates a film entirely formed with irregular patterns.                

Bismaleimides were added to the composition of Example 2 and to thecomposition of Example 3 and were stirred at room temperature todissolve, and films were prepared in the same manner as above from theresins obtained. The films were checked for heat resistance with theresults given in Table 2.

                  TABLE 2                                                         ______________________________________                                                              Heat resistance                                         Resin           Bismaleimide                                                                              260° C.                                                                        280° C.                            ______________________________________                                        Example 4       A.sup.*4 17 wt. %                                                                         a       a-b                                       (Composition of Ex. 2)                                                        Example 5                                                                     (Composition of Ex. 2)                                                                        A 30 wt. %  a       a                                         Example 6                                                                     (Composition of Ex. 3)                                                                        A 30 wt. %  a       a                                         Example 7                                                                     (Composition of Ex. 2)                                                                        B.sup.*5 17 wt. %                                                                         a       a-b                                       Example 8                                                                     (Composition of Ex. 3)                                                                        B 17 wt. %  a       a-b                                       Blank HI-400      --        b       c                                         ______________________________________                                         Note:                                                                         ##STR2##                                                                      ##STR3##                                                                 

What we claim is:
 1. A heat resistant resin composition comprising thereaction product of (1) a polyamide imide resin having carboxyl groupsproduced from at least one aromatic polycarboxylic acid anhydride havingat least three functional carboxyl groups and at least one organicdiisocyanate and (2) a compound selected from the group consisting ofN-methylolated acryalamide, N-methoxymethylacrylamide,N-butoxymethylacrylamide, N-methylolated methacrylamide andN-butoxymethylmethacrylamide, said compound (2) being used in an amountof 1.0 to 5.0 mole equivalent weights per mole equivalent weight of thecarboxyl groups in the polyamide imide resin.
 2. The heat resistantresin composition of claim 1 wherein said polyamide imide resin havingcarboxyl groups contains at least 4 carboxyl groups per molecule.